This invention relates to a test arrangement which is suitable for performing electrical measurements on high frequency electrical devices. The invention is intended for operation in the radio frequency or microwave frequency range.
One of the most powerful techniques used to characterise radio frequency (R.F.) and microwave components is the Scattering Parameter (or S-Parameter) model. The R.F. device is treated as a "black box" with various input and output terminals. Each pair of terminals is termed a "port". The device's performance is quantified by applying a wave to one port and recording the emergent waves at each port caused by that wave. The relationships between the waves are expressed in a matrix of complex coefficients, called the S-parameter matrix.
These complex coefficients can be measured by means of a network analyser which separates out the required waves and evaluates their complex relationship. Automatic network analysers have been developed in which the frequency of a variable frequency oscillator is controlled over a particular frequency band. These analysers are very complex involving directional couplers or bridges to sample the incident and reflected waves. The samples are than ratioed to find the magnitude and phase of the reflection coefficient. This is a complicated step in practice, as phase angles cannot be directly measured at frequencies higher than about 500 kHz. Therefore, the system has to convert the RF samples down to a suitable frequency before measuring them. To do this while accurately maintaining their amplitude and phase relationships is both difficult and expensive. Mixers or samplers used in such a heterodyne system must be extremely linear.
An arrangement known as a Six-Port Network Analyser relies on the use of amplitude measurements, without the need to measure phase information, and is able to use relatively simple RF circuitry, providing that the Six-Port Network Analyser can be accurately calibrated. As previously known, such arrangements have severe operational drawbacks; they are slow and cumbersome to use, and are regarded primarily as laboratory instruments requiring the use of skilled operators. To fully characterise a device under test (DUT) the use of a dual configuration of Six-Port Network Analysers is needed. It can be shown that the DUT must be such that its characteristics are independent of power applied to both ports simultaneously. This is generally true only for passive devices. In particular, measurements of amplifiers would be unreliable because of the unrepresentative operating conditions. There is a strong possibility that the amplifier could oscillate instead of amplify. Broadband operation is not possible because directional isolators are needed to prevent the RF source signal circulating around a signal loop which includes the DUT.
The present invention seeks to provide an improved test arrangement.